Support is requested to continue research into the hormonal control of behavioral and neuroendocrine sexual differentiation in the ferret. Studies are proposed to (a) compare the plasma concentrations of sex steroids in male and female ferrets killed at several prenatal ages and at several times within 24h after birth, (b) determine whether testosterone (T) acts prenatally or immediately after birth to sensitize the male ferret brain to the masculinizing action of T on sexual behavior, which occurs postnatally, (c) determine whether this perinatal process of brain sensitization to the later effect of T depends on the neural action of androgen versus estrogen, (d) determine whether the completion of the process of coital masculinization can be advanced significantly in male ferrets by prenatal exposure to pharmacological dosages of T, (e) determine whether aromatase activity within different regions of the preoptic area/anterior hypothalamus (POA/AH), as opposed to other subcortical brain regions, is greater in male than in female ferrets prenatally when the sexually dimorphic dorsal nucleus (DN) of the POA/AH is organized in males, (f) determine whether electrolytic or neurotoxic lesions of cells in the sexually dimorphic DN POA/AH attenutates the androgenic activation of masculine sexual behavior or accentuates the estrogenic activation of proceptive behavior in adult male ferrets, (g) determine whether stressing pregnant ferrets enhances the ability of offspring to express receptive behavior in the absence of concurrent estrogen, and (h) determine whether the behavioral and endocrine control of luteinizing hormone secretion in the ferret is sexually dimorphic.